I just got the blank deck for Once upon a time Second edition.
I can find no guide for how I should balance the new cards. How many interrupts should I make, how many events, places, objects and aspects and so on?
Thank you.
The short answer is that you can do whatever you want. Although the game works well with the balance of card types provided, part of the creativity that the blank cards offer is the opportunity to change that balance if you want.
Obviously, though, the core set was balanced through playtesting, and works very well with that mix. If you'd like to keep to that balance because it's known to be optimized, you should remove cards from your core game to match the types and quantities that you add. The reason is that this will keep the balance of interrupts to non-interrupts the same as the balance among the card types. (Make sense?)
Thanks again, and thanks for picking up the Create-Your-Own Cards!
Thank you very much for the answer.
Technically, would that balance be maintained if I worked out the percentages and made sure that the deck keeps as close as possible to the same ratio between the different cards?
This kind of game feels like it would be hard to balance though, I am planning a space opera expansion, I am a bit scared that most of it's tropes that you really want are way too epic. I want to keep as many cards as possible that doesn't completely conflict with the genre to make sure there is a balance, and while there could certainly be Swords and Horses in Space opera, just look at Star Wars and Firefly respectively, I still kind of don't want them there. Maybe that's just my preference though. They are definitely harder to work into the story in a natural way.
I just had a thought that maybe I should work the most epic concepts into the ending cards. Do you think that would make them too specific? Some of the endings are more specific than I like already though, but maybe that should be part of the challenge?
If you're interested in creating all (or most of) a complete play set with the Create-Your-Own cards, yes, absolutely, I'd stick to about the same proportions of the suits (with respect to each other), and about the same proportions of interrupts with respect to non-interrupts.
When creating cards, you'll want to create cards that are not too specific (so that it will be hard to imagine how that trope fits into most stories), and not too general (so that it's impossible to tell any story in that genre without triggering someone's opportunity to interrupts you with that card). It's a bit of an art, and playtesting is key to getting the set to where you want it. The good news is that the process of iterating and testing is fun. 
A good mix of specificity and generality is also good in the endings. The best ending will require specific lead-ups in the telling of the main body of the tale, so that you've got to plan in advance to work your way toward your own unique ending.
Good luck!
question in topic. once you know all the secrets, do you still enjoy playing as much?